Research is directed toward an understanding of the ionic mechanisms mediating cell function and synaptic transmission between cells in the vertebrate retina. These studies are an outgrowth of earlier work which described the functional organization of the retina, based upon the light response activity of each cell type. Knowing what the cells do under various conditions, we are now trying to understand how they do it in terms of levels of transmitter release, changes in ionic channel conductance, and electrotonic transmission between cells. Most of our studies rely upon a thorough understanding of the electrical state of the membrane in the dark, and the alteration in that state in the presence of light, synaptic blockade, or application of mimetic substances. Retinal cells of each type have been studied with pairs of electrodes placed intracellularly to define the current-voltage characteristics. Specific projects for the coming year are: study of the feedback synapse from horizontal cells to cones, study of the time-dependent conductance changes in photoreceptors, study of electrotonic coupling between rods and between cones, study of electrical and chemical transmission between horizontal cells, identification of the photoreceptor transmitter, and others.